1. Field of the Invention
The present invention relates to a color Braun tube apparatus.
2. Description of the Related Art
Generally, as shown in FIG. 17, a color Braun tube apparatus has an envelope in which a panel 1 is integrally connected to a funnel 2. On an inner surface of the panel 1, a phosphor screen 4 composed of phosphor layers of three colors emitting red, green, and blue light is formed, and a shadow mask 3 with a number of electron beam passage apertures formed thereon is attached to an inner wall surface of the panel 1 so as to be opposed to the phosphor screen 4. An electron gun 6 is provided in a neck 5 of the funnel 2, and a deflection yoke 8 is mounted on an outer circumferential surface of the funnel 2. Three electron beams 7B, 7G, and 7R emitted from the electron gun 6 are deflected by a magnetic field generated by the deflection yoke 8 to scan the phosphor screen 4 in a horizontal direction and a vertical direction, whereby a color image is displayed on the phosphor screen 4.
Such a color Braun tube apparatus generally is an in-line type color Braun tube apparatus. The in-line type color Braun tube apparatus uses, as the electron gun 6, an in-line type electron gun emitting three electron beams in an in-line shape so that a center electron beam (hereinafter, referred to as a “center beam”) at the center and a pair of side electron beams (hereinafter, referred to as “side beams”) on both sides of the center beam are aligned on an identical horizontal plane, in which the magnetic field generated by the deflection yoke 8 is set to be a non-uniform magnetic field with a horizontal deflection magnetic field being a pin-cushion type and a vertical deflection magnetic field being a barrel type, whereby the three electron beams are self-converged.
As the in-line type electron gun, various kinds of systems are known, and one of them is a Bi-Potential Focus (BPF) type Dynamic Astigmatism and Focus correction (DAF) system.
Regarding a main lens configuration of the in-line type electron gun, various kinds of types also are known, and one of them is a superimposed electric field type.
FIGS. 18A and 18B show an example of the in-line type electron gun. FIG. 18A is a horizontal cross-sectional view thereof, and FIG. 18B is a vertical cross-sectional view thereof
The electron gun is composed of three cathodes K arranged in a line in a horizontal direction, and a first grid G1 to a fourth grid G4 with an integrated configuration, arranged successively on a phosphor screen side with respect to the three cathodes K.
In the first grid G1 and the second grid G2, three electron beam passage apertures corresponding to the three cathodes K arranged in a line are formed respectively.
The (3-1)th grid G3-1 is composed of a tubular body in which three electron beam passage apertures are formed respectively on both end faces.
The (3-2)th grid (focus electrode) G3-2 is composed of a tubular body 10 in which three electron beam passage apertures are formed on an end face on an electron beam incident side (i.e., a surface opposed to the (3-1)th grid G3-1), and an oval electron beam passage aperture 9 common to the three electron beams, having a major axis in the arrangement direction of the three electron beams as shown in FIG. 19, is formed on an end face on an electron beam output side (i.e., a surface opposed to the fourth grid G4). In the (3-2)th grid G3-2, an electric field correcting plate 12 is placed in which three electron beam passage apertures 11B, 11G, 11R are formed.
The fourth grid (anode electrode) G4 is composed of a tubular body 14 in which an oval electron beam passage aperture 13 common to the three electron beams, having a major axis in the arrangement direction of the three electron beams as shown in FIG. 20, is formed on an end face on an electron beam incident side (i.e., a surface opposed to the (3-2)th grid G3-2). In the fourth grid G4, an electric field correcting plate 16 is placed in which three electron beam passage apertures 15B, 15G, 15R are formed.
The cathodes K are supplied with a voltage of about 150 V, the first grid G1 is grounded, and the second grid G2 is supplied with a voltage of about 600 V. The (3-1)th grid G3-1 is supplied with a focus voltage of about 8 kV, and the (3-2)th grid G3-2 is supplied with a dynamic focus voltage that increases in accordance with the deflection distance of the electron beams on a base of about 8 kV. The fourth grid G4 is supplied with a high voltage of about 30 kV.
Thus, the cathodes K, the first grid G1, and the second grid G2 constitute a tripolar part for generating electron beams and forming an object point with respect to a main lens (described later). The second grid G2 to the (3-1)th grid G3-1 form a prefocus lens for preliminary focusing the electron beams emitted from the tripolar part. When the electron beams are deflected, the (3-1)th grid G3-1 and the (3-2)th grid G3-2 form a quadrupole lens that has a focusing function in a horizontal direction and has a diverging function in a vertical direction. Furthermore, the (3-2)th grid G3-2 that is a focus electrode and the fourth grid G4 that is an anode electrode form a superimposed electric field type BPF main lens for finally accelerating and focusing the electron beams with respect to the phosphor screen.
In the above-mentioned electron gun, in the case where the electron beams travel to the center of the phosphor screen without being deflected, the quadrupole lens is not formed between the (3-1)th grid G3-1 and the (3-2)th grid G3-2. The electron beams from the tripolar part are preliminary focused by the prefocus lens, and then, focused at the center of the phosphor screen by the main lens.
In contrast, in the case where the electron beams are deflected to a circumferential portion of the phosphor screen, the voltage of the (3-2)th grid G3-2 increases in accordance with the deflection amount of the electron beams, and a quadrupole lens for focusing the electron beams in a horizontal direction and diverging them in a vertical direction is formed between the (3-1)th grid G3-1 and the (3-2)th grid G3-2. Simultaneously, the increase in the voltage of the (3-2)th grid G3-2 decreases the lens strength of the main lens formed by the (3-2)th grid G3-2 and the fourth grid G4. This corrects the deflection aberration generated by the enlargement of the distance between the main lens and the phosphor screen caused by the deflection of the electron beams, and the non-uniform electric field containing a pin-cushion type horizontal deflection magnetic field and a barrel type vertical deflection magnetic field generated by the deflection yoke.
In order to make the image quality of the color Braun tube satisfactory, it is necessary to make the focus characteristics on the phosphor screen satisfactory, i.e., to reduce the size of an electron beam spot over the entire surface of the phosphor screen.
As one means for reducing the size of an electron beam spot, there is a method for enlarging a main lens aperture of the electron gun. The above-mentioned superimposed electric field type main lens in the BPF-type electron gun generally is used as a configuration in which a lens with a larger aperture compared with that of a simple cylindrical lens is obtained.
However, since the electrode size is limited by the inner diameter of a neck, there is a limit to a lens aperture that can be formed, even with the superimposed electric field type main lens. JP8(1996)-22780A and JP9(1997)-180648A describe a method for obtaining a lens aperture larger than that of the superimposed electric field type main lens. FIGS. 21A and 21B show an exemplary case where this method is applied to the above-mentioned BPF-type DAF system electron gun. FIG. 21A is a horizontal cross-sectional view thereof, and FIG. 21B is a vertical cross-sectional view thereof.
The electron gun is different from the conventional electron gun forming a superimposed electric field type main lens shown in FIGS. 18A and 18B, in that an intermediate electrode GM is placed between the (3-2)th grid G3-2 and the fourth grid G4, and is supplied with a voltage higher than the third grid voltage (focus voltage) and lower than the fourth grid voltage (anode voltage), which is obtained by dividing an anode voltage with a resistor 17 housed in a tube. As shown in FIG. 22, the intermediate electrode GM is composed of a tubular body 20 in which an electron beam passage aperture 18 common to the three electron beams is formed on a surface opposed to the (3-2)th grid G3-2 that is an incident side surface of the electron beams, and an electron beam passage aperture 19 common to the three electron beams is formed on a surface opposed to the fourth grid G4 that is an output side surface of the electron beams, and an electric field correcting plate 22 having three electron beam passage apertures 21B, 21G, 21R placed inside the tubular body 20.
According to the above-mentioned electron gun, a main lens with an aperture larger than that of the superimposed electric field type main lens can be formed, so that an electron beam spot can be reduced in size, which can enhance the resolution of a color Braun tube.
However, when the electron beam spot is reduced in size, although the resolution is enhanced, moire is likely to be generated on a screen due to the interference between the scanning lines and the shadow mask, which rather impairs image quality. This is caused by the extremely small vertical dimension of the electron beam spot.
Furthermore, as the main lens aperture is larger, the change in a focal length with respect to a focus voltage decreases, which makes it necessary to increase further the amplitude of a dynamic focus voltage in the DAF system electron gun. This increases the cost of a driving circuit and decreases withstand voltage reliability.
That is, it is desirable that while the aperture of a main lens in a horizontal direction is as large as possible, the aperture thereof in a vertical direction is set appropriately. More specifically, the aperture in a vertical direction preferably is about 5 to 9 mm. However, a desirable main lens cannot be formed with the prior art.